1. Field of the Invention
The present invention relates to a scanning electronic beam apparatus for detecting the secondary electrons given off from a sample in response to scanning the sample with an electron beam using a plurality of secondary electron detectors to process the signals from the detectors thereby making possible sample observation.
2. Description of the Related Art
Various electronic beam apparatuses, such as scanning electronic microscopes, have been conventionally used in order to perform topological inspections or observations of microfabricated electronic devices. In particular, there is an increasing need for observation with resolution effective for obtaining stereoscopic detection of samples, due to the advancement of super micro-fabrication in recent electronic devices. There is a known method using the secondary electrons from a sample as well as a plurality of detectors, as a method for SEM-observing a concave/convex shape on a sample surface (see JP-B-40-17999).
This conventional method is as follows. As shown in FIG. 10, first the electron beam 101 converged in diameter by a lens is deflected by a deflector unit 103, to conduct scanning over a surface of a sample 102. The reflection electrons caused from the sample 102 and back-scattered electrons 105 comprising secondary electrons are detected by the detectors 104a, 104b arranged nearly symmetrically about a sample normal line. If an electron beam is scanned in an arrowed direction onto a sample comprising different elements A, B shown in FIG. 11A, the signal to be detected by the detector 104a varies as in FIG. 11B while the signal to be detected by the detector 104b varies as in FIG. 11C. The detection signal resulting from a slant surface given from the detector 104b opposed to the slant surface is greater than the detection signal by the detector 104a. The detection signal outputs, if subtracted by a subtraction circuit 106, provides a signal dependent only on a concave/convex as shown in FIG. 11D. If this is further integrated by an integration circuit 107, an output can be obtained that nearly equals to the concave/convex on the sample surface as shown in FIG. 11E. Thus, the concave/convex topology on the sample surface can be observed on a display unit 108, wherein an image with greater stereoscopic feeling is obtained.
However, resolving power can be improved greater in an objective lens when a sample is placed in an intense lens magnetic field than in the usual objective lens when a sample is not put in a magnetic field. However, the secondary electrons given off from the sample will not travel toward the detector as in FIG. 10, i.e. helically moved around the optical axis a multiplicity of time and swirled up toward an electron source by the intense objective-lens magnetic field to a region free of the lens magnetic field where they are scattered in various directions. Accordingly, on a plane of the detector provided close to the electron source, there is a difficulty in distinguishing a directionality of the secondary electron upon being given off from the sample.
Namely, the conventional single-pole magnetic field type objective lens of a scheme to place a sample in an intense lens magnetic field has a structure, e.g. disclosed in JP-A-3-1432 (U.S. Pat. No. 5,023,457), as shown in FIG. 12. According to this structure, the secondary-electron beam 203 caused from a sample 202 due to the primary electrons emitted from the electron source and incident in a direction of an optical axis 201 (Z (mm)) travels toward the electron source and is detected by a secondary-electron detector 204.
FIG. 13 shows a simulation example of a secondary electron beam path in the lens (sample position is Z=xe2x88x928 mm and top-surface position of a single magnetic pole 205 is Z=0). The secondary electrons helically move along an optical axis (Z) in a range having a great lens magnetic field (xe2x88x928 mm xe2x89xa6Zxe2x89xa6≈0 mm) and scatter in various directions in a range the magnetic field vanishes (≈0 mm  less than Z). From FIG. 13, it can be seen that the secondary electrons given off leftward on the sample and the secondary electrons given off rightward on the sample are mixed together thus lose the directionality thereof upon leaving the sample.
In this manner, in the conventional apparatus there is difficulty in specifying the directionality of the secondary electrons given off from the sample because they are mixed together. Thus, there is a problem that, even if the secondary electrons passed the objective lens are detected by a plurality of detectors provided close to the electron source symmetrically about the optical axis to operate the secondary-electron signals from the detectors, there is difficulty in effectively observing the concave/convex topology of a sample.
It is an object of the present invention to provide a scanning electronic beam apparatus capable of solving the foregoing problem in the related art.
In order to detect secondary electrons given off from a sample by a plurality of detectors and obtain a concave/convex image of the sample or a high-resolution image emphasized in stereoscopic detection, the present invention provides a scanning electronic beam apparatus using an objective lens in which a sample is placed within an intense 7 lens magnetic field, and the lens has a pair of electrodes comprising upper and lower electrodes having electrode apertures for passing an electron beam to a lens magnetic pole tip of the objective lens to apply a negative voltage to the sample and the lower electrode opposed thereto, whereby a zero or positive voltage is applied to the upper electrode provided close to an electrode source to cause an electric field for accelerating the secondary electron from the sample to an objective-lens magnetic field space closer to the electron source thereby suppressing the helical motion thereof.
With this structure, the secondary electron caused from a sample due to electron-beam irradiation, after passing through the electrode aperture of the lower electrode and the electrode aperture of the upper electrode, can be incident with directionality on the plurality of secondary electron detectors arranged substantially symmetrically around the optical axis. As a result, by carrying out an operation process on the basis of the signals from the detectors, it is possible to favorably observe a concave/convex image of the sample and obtain a high-resolution image emphasized in stereoscopic feeling.
According to the present invention, there is proposed, in a scanning electronic beam apparatus adapted to place a sample within a magnetic field by an objective lens built, at a lens magnetic pole tip, with a pair of electrodes along an optical axis, the scanning electronic beam apparatus characterized in that: a negative voltage is applied to a sample and one electrode opposed thereto while a zero or positive voltage is applied to the other electrode, thereby causing an electric field to suppress helical motion of secondary electrons given off from the sample due to electronic-beam irradiation in a region of from the sample to an objective-lens magnetic field space close to an electron source.
Electrode apertures for passing the secondary electron may be respectively provided in said one and the other electrodes to detect the secondary electron passed through said electrode apertures by a plurality of secondary electron detectors arranged substantially symmetrically around the optical axis.
Electrode apertures for passing the secondary electron may be respectively provided in said one and the other electrodes to cause the secondary electron passed through said electrode apertures to be incident on a reflection plate and detect an electron caused from said reflection plate by a plurality of secondary electron detectors arranged around the optical axis.
The reflection plate may have a partition plate.
An observation image of the sample may be obtained by operation on the basis of signals from said plurality of secondary electron detectors.
Values of negative voltages applied to the sample and said other electrode may be substantially equal.
The other electrode is a magnetic member.
According to the invention, there is also proposed, in a scanning electronic beam apparatus having an objective lens provided with an electrode opposed to a sample to place the sample within a lens magnetic field of said objective lens, the scanning electronic beam apparatus characterized in that: a negative voltage is applied to the sample and said electrode is put at a potential higher than the negative voltage, thereby causing an electric field to suppress helical motion of secondary electrons given off from the sample due to electronic-beam irradiation from the sample to an objective-lens magnetic field space close to an electron source.
An electrode aperture for passing the secondary electron may be provided in the electrode to detect the secondary electron passed through the electrode aperture by a plurality of secondary electron detectors arranged substantially symmetrically around an optical axis.
An electrode aperture for passing the secondary electron may be provided in the electrode to cause the secondary electron passed through the electrode aperture to be incident on a reflection plate and detect an electron caused from the reflection plate by a plurality of secondary electron detectors arranged around an optical axis.
The reflection plate may have a partition plate.
An observation image of the sample may be obtained by operation on the basis of signals from the plurality of secondary electron detectors.
The other electrode may be a magnetic member.
According to the invention, there is also proposed, in a scanning electronic beam apparatus using an objective lens of a scheme to place a sample within a lens magnetic field, the scanning electronic beam apparatus characterized in that: a negative voltage is applied to the sample to cause, in a space to an opposed magnetic pole at a ground potential to the sample, an electric field to suppress helical motion of secondary electrons given off from the sample due to electronic-beam irradiation in a region of from the sample to an objective-lens magnetic field space close to an electron source.
The secondary electron caused from the sample due to the electron-beam irradiation, after passing through a single magnetic pole aperture, may be detected by a plurality of secondary electron detectors arranged around an optical axis.
The secondary electron caused from the sample due to the electron-beam irradiation, after passing through a single magnetic pole aperture, may be incident on a reflection plate to detect an electron caused from the reflection plate by a plurality of secondary electron detectors arranged around an optical axis.
The reflection plate may have a partition plate.
An observation image of the sample may be obtained by operation on the basis of signals from the plurality of secondary electron detectors.